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There have been several approaches in order to explain the pressure induced first-order phase transition connected with the discontihuous change of the occupation of the 4f state a s observed in SmS. Most of them were based on the purely electronic Falicov-Kimball model extended by .hybridization (see /1 to 6/ and earlier works cited therein). However, the results derived in /1/ and /6/ seem to suggest that one should rather elucidate the role of phonons in order to explain the problem as.proposed in /7/ within the Hartree-Fock approximation. It has been found /7/ that the phonon induced f-d transitions are necessary for the discontinuouk change of valence which has been confirmed by d e Menezes and Troper /8/. This remains in some contradiction with recent results of Misawa and Takano /9/ who concluded that ordinary coupling of felectrons with the phonon mode can produce the transition. Thus, the type of